When carrying out process engineering operations, it is frequently necessary to contact a liquid with a gas, referred to as the process gas, for the purpose of mass transfer. The conditions arising in these processes are very diverse. Accordingly, there are also numerous possible design variants. Generally, the gassing equipment must meet the following requirements:
high absorption efficiency PA1 operational reliability PA1 low hydraulic head PA1 small constructional volume PA1 low investment costs PA1 low operating costs PA1 variable throughput without significant losses in efficiency. PA1 blowing ozone-containing gas in by means of a special turbine PA1 blowing ozone-containing gas in through porous pipes or bodies PA1 blowing ozone-containing gas in by means of an injector.
When gassing a liquid, the gas is dispersed into small bubbles with the aid of a suitable apparatus and is then dissolved in the liquid in a reactor section, referred to as the bubble column. The required residence time in the reactor section mainly depends on the maximum initial bubble radius and on the rate of dissolution. In practice, the gas-dispersing apparatus must meet the requirement that the maximum bubble radius does not exceed a defined (very small) size. Otherwise, the required residence time and the reactor section become very long, and the process becomes uneconomical. This applies in particular to processes in which a liquid is to be treated with ozone or an ozone-containing gas, since a comparatively large energy must be supplied in the generation of ozone, and unconsumed ozone can be recovered only with great difficulty.
In the known types of equipment for gassing liquids with ozone, essentially the following processes are used:
In the first mentioned process, the water to be treated is introduced into the suction zone of a turbine which has a special profile and which forces the water downwards against a stream of ozonized air which is blown in below the turbine. In the bubble column zone close to the turbine, a fine dispersion zone (ozonized air/water) is created. The finely dispersed ozonized air/water mixture is again passed through the turbine, the throughput of the turbine being a multiple of the quantity of water to be treated. After this mixing, the gas/water emulsion rises in a second unit where the contact continues. In this way, bubble lives of more than one minute can be realized (op. cit., page 243, FIG. 167).
In the second process, porous pipes are fitted in the lower part of a bubble column, the ozonized air flowing out of these pipes in very small bubbles. The water to be ozonized flows into the upper part of the bubble column. Intimate counter-current contact of the two media is thus achieved. The bubble columns can also be designed with several units, part injections of ozonized air likewise taking place preferably in counter-current. The residence time of the bubbles and hence the absorption efficiency, however, is here far lower than that of the turbine mixing process (op. cit., page 243, FIG. 166).
In the third process--blowing the ozonized air in by means of an injector--there are two alternatives to be distinguished:
If a water pressure of at least 2 m water gauge is available, an ejector can be operated by this pressure. In this case, the total quantity of water to be treated is blown through the ejector, which at the same time aspirates the ozonized air and feeds the bubble column from below (op. cit., page 244, FIG. 168).
If the available pressure is less than 2 m water gauge, the quantity of water to be treated is divided into two unequal parts. The pressure of the smaller part is raised by means of a pump, so that the ejector for the ozonized air can be operated. The remaining, unozonized throughput is passed with the natural gradient into the lower part of the bubble column. The effectiveness of this method leaves something to be desired, since the two parts of water are subjected to different treatments and the distribution of ozone is thus not homogeneous.